Astronomers soon realized that the companion star passes directly in front of the black hole on its three-day orbit, eclipsing the black hole's x-ray emissions.

This arrangement allowed the team to combine data from NASAs orbiting Chandra X-Ray Observatory and the Gemini North Telescope on Mauna Kea, Hawaii, to calculate the two object's masses more accurately than usual.

Orosz and his team don't fully understand how a massive black hole formed in close orbit around a massive companion star.

But experts think the formation of close binaries that contain the remains of dead stars—such as white dwarfs, neutron stars, and black holes—might involve a process called the common-envelope phase.

During this phase, a dying star in a binary system expells its outer layers while sucking the other star closer.

This leads to either a merger of the two objects or the formation of a tight binary system.

"During the common-envelope phase, the progenitor of the M33 X-7 black hole must have lost a large amount of mass for the two objects to be so close," Tomasz Bulik, of the University of Warsaw in Poland, writes in a commentary article also appearing in Nature.

"In this system we know that quite a lot of mass was expelled, and on the other hand that the remaining mass formed the black hole," Bulik told National Geographic News.

"Thus we have an upper and lower bound on the amount of mass expelled."

According to lead study author Orosz, "the reason M33 X-7 is so hard to understand is that both the black hole and the companion star have a very high mass.

"The predicted mass loss [of the star as it died before becoming a black hole] is so great that M33 X-7 should not have been able to enter a common-envelope phase."

Researchers hope that by understanding systems like M33 X-7, they can gain better insight into the formation of tight binary systems.

"The common-envelope stage is poorly known," Bulik said, "and this system will help us understand it."

Also this week, an independent team led by Andrea H. Prestwich of the Harvard-Smithsonian Center for Astrophysics described what it says could be an even more massive stellar black hole weighing 24 to 33 times as much as the sun.

The object is also part of a binary system that includes a massive star, and it lies 2.2 million light-years away in the starburst galaxy IC 10.

The researchers, who published their findings online this week in the Astrophysical Journal Letters, say more data are needed to confirm that the object is actually a black hole.